SCBF Over Moment Frame

[cal91]

I have a V brace that's over a Moment Frame. My question is: Does the the W18X86 beam need to be designed for the unbalanced force of the 2L3.5X3.5X1/2 braces (vertical component = expected tension - expected post buckled compression).

If it was a normal beam, then I wouldn't even be asking this question. That beam must stay elastic during the design earthquake.

However, this is a SMF beam, which is designed to yield during the earthquake. I don't think it's logical to design the beam for the maximum expected forces from the brace, and then to design the moment connection/column for the maximum expected forces from the beam. Rather it makes sense to design the braces and the moment frame beam (the "fuses") for the unamplified seismic loads, and then to design the connections, columns,and non-moment frame beams for the amplified load.

Thoughts?

Note: If I could have the braces be Chevron I would, but for reasons I won't get into I can't go there.

 

[KootK]

Note: If I could have the braces be Chevron I would, but for reasons I won't get into I can't go there.

Totally headed me off at the pass there.

Thoughts?

I think you do some neat stuff.

To form a complete mechanism here, you've got to a) yield those braces and b) yield the MF beam ends and c) Keep everything else intact so that it can enjoy the ride. Let's start with that.

So it seems to me that, rationally, your beam needs to survive a load case that looks like this:

- Overstrength plastic hinging moments at the ends.

- Whatever gravity loads make sense for the load casing.

- An unbalanced force from the bracing above equal to the greater of a) force assuming over strength brace yield or b) amplified brace force.

Do you have the Paulay & Priestly book on seismic design of concrete an masonry? They cover a moment frame beam design example with a substantial point load on the beam. I see that as being substantially similar to this. It's a bit of an accounting headache. Hopefully we get some help from sandman on this. He's pretty good at this kind of thing, especially the code particulars.





I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Agent666]

If it were me, I'd design the L1 collector beam similar to the ones above for the bracing loads, and locate your ground floor storey 'portal(s)' in the other bays, separate the interaction between the two systems goes a long way to simplifying things in terms of achieving dependable behaviour and making your life as a designer simplier.

Watch out for creating a soft storey, whereby your entire buildings drift accumulates in the first storey due to the portal mechanism forming (presumably hinges in the beam) and the upper levels simply translating as one rather rigid block. You will rapidly exceed any allowable rotational limits in the beam hinge regions.
I'd design it as being elastic for this reason if the forces allow for it, otherwise you need to carefully think about how and what happens when the ductile mechanism forms. Your braces look pretty slender so maybe your forces are low enough to be able to accommodate this approach, making the lowest level elastic, in effect capacity design protecting it?

 

[sandman21]

I dont recall the code having anything specific regarding this condition. I think we can infer based on the desired behavior of a SMF and beam bracing requirements a rational approach for analyzing the system. AISC 341 E3.2 basis of design the provisions for SMF "...are expected to provide significant inelastic deformation capacity through flexural yielding..." of the beams. We then design the beams, connections and column to force the yielding in the location we choose. We brace the plastic hinge and ensure that no changes in the member occur to ensure we develop the moments. AISC 341-16 E3.4b added a section requiring beam bracing at "...concentrated forces, changes in cross section, and other locations where analysis indicates that a plastic hinge will form...". I see nothing that would preclude the use of the system framing into the SMF beam. The SCBF requirement for the unbalanced loading design is because as the building rides out the earthquake you lose compression capacity each cycle. The maximum load will not occur on the first movement or the second movement, but later in the earthquake. At the same time you are forming plastic hinges at the ends of the SMF beam. With the load the SCBF imparts on the beam you will be forming a third plastic. I would be designing the beam for the unbalanced loading to ensure I only have at most two hinges in the beam.